NitroCop

Fast Ruby linter in Rust targeting RuboCop compatibility.

Note

🚧 Early-stage: Detection is high-fidelity on most codebases but edge cases remain. Autocorrect is not yet complete. Expect bugs.

Benchmark on the rubygems.org repo (1,222 files), Apple Silicon:

Scenario	nitrocop	RuboCop	Speedup
Local dev (50 files changed)	64ms	1.39s	21.7x
CI (no cache)	207ms	18.21s	87.8x

Features

• 915 cops from 6 RuboCop gems (rubocop, rubocop-rails, rubocop-performance, rubocop-rspec, rubocop-rspec_rails, rubocop-factory_bot)
• 95.1% conformance against RuboCop across 1,000 open-source repos
• Autocorrect (-a/-A) is partial — work in progress
• Reads your existing .rubocop.yml — no migration needed
• Uses Prism (Ruby's official parser) via ruby-prism crate
• Parallel file processing with rayon

Quick Start (Work in progress 🚧)

Requires Rust 1.85+ (edition 2024).

cargo install nitrocop   # not yet published — build from source for now

Then run it in your Ruby project:

nitrocop

Configuration

nitrocop reads .rubocop.yml with full support for:

• inherit_from — local files, recursive
• inherit_gem — resolves gem paths via bundle info
• inherit_mode — merge/override for arrays
• Department-level config — RSpec:, Rails: Include/Exclude/Enabled
• AllCops — NewCops, DisabledByDefault, Exclude, Include
• Enabled: pending tri-state
• Per-cop options — EnforcedStyle, Max, AllowedMethods, AllowedPatterns, etc.

Config auto-discovery walks up from the target directory to find .rubocop.yml.

Cops

nitrocop supports 915 cops from 6 RuboCop gems:

Gem	Version	Cops	Coverage	Departments
rubocop	1.84.2	593	100%	Layout, Lint, Style, Metrics, Naming, Security, Bundler, Gemspec, Migration
rubocop-rails	2.34.3	138	100%	Rails
rubocop-performance	1.26.1	52	100%	Performance
rubocop-rspec	3.9.0	113	100%	RSpec
rubocop-rspec_rails	2.32.0	8	100%	RSpecRails
rubocop-factory_bot	2.28.0	11	100%	FactoryBot

Every cop reads its RuboCop YAML config options and has fixture-based test coverage.

Conformance

We diff nitrocop against RuboCop on 1,000 open-source repos (231k Ruby files) with all cops enabled. Every offense is compared by file, line, and cop name.

	Count	Rate
Agreed	11.4M	95.2%
nitrocop extra (FP)	63.2K	0.5%
nitrocop missed (FN)	514.5K	4.3%

Per-repo results (top 15 by GitHub stars):

Repo	.rb files	RuboCop offenses	nitrocop extra (FP)	nitrocop missed (FN)	Agreement
rails	3,498	314,853	967	17,139	94.2%
jekyll	190	13,052	75	737	93.8%
mastodon	3,123	76,303	108	2,550	96.5%
huginn	451	34,402	154	961	96.7%
discourse	9,172	619,551	2,186	13,691	97.4%
fastlane	1,302	118,730	200	3,012	97.3%
devdocs	833	19,903	118	1,283	93.0%
chatwoot	2,262	64,941	59	1,496	97.6%
vagrant	1,460	86,064	217	2,946	96.3%
devise	206	5,800	20	411	92.5%
forem	3,390	128,530	262	4,330	96.4%
postal	294	13,948	24	715	94.7%
CocoaPods	438	28,422	270	1,944	92.2%
openproject	9,286	388,980	645	10,574	97.1%
gollum	55	3,790	21	295	91.7%

Remaining gaps are mostly in complex layout cops (indentation, alignment) and a few style cops. See docs/corpus.md for the full corpus breakdown.

Hybrid Mode

Use --rubocop-only to run nitrocop alongside RuboCop for cops it doesn't cover yet:

#!/usr/bin/env bash
# bin/lint — fast hybrid linter
nitrocop "$@"

REMAINING=$(nitrocop --rubocop-only)
if [ -n "$REMAINING" ]; then
  bundle exec rubocop --only "$REMAINING" "$@"
fi

CLI

nitrocop [OPTIONS] [PATHS]...

Arguments:
  [PATHS]...    Files or directories to lint [default: .]

Options:
  -a, --autocorrect         Autocorrect offenses (safe cops only)
  -A, --autocorrect-all     Autocorrect offenses (all cops, including unsafe)
  -c, --config <PATH>       Path to .rubocop.yml
  -f, --format <FORMAT>     Output format: text, json [default: text]
      --only <COPS>         Run only specified cops (comma-separated)
      --except <COPS>       Skip specified cops (comma-separated)
      --rubocop-only        Print cops NOT covered by nitrocop
      --stdin <PATH>        Read source from stdin, use PATH for display
      --debug               Print timing and debug info
      --list-cops           List all registered cops
      --ignore-disable-comments  Ignore all # rubocop:disable inline comments
      --cache <true|false>  Enable/disable file-level result caching [default: true]
      --cache-clear         Clear the result cache and exit
      --init                Resolve gem paths and write lockfile to cache directory, then exit
      --fail-level <SEV>    Minimum severity for non-zero exit (convention/warning/error/fatal)
  -F, --fail-fast           Stop after first file with offenses
      --force-exclusion     Apply AllCops.Exclude to explicitly-passed files
  -L, --list-target-files   Print files that would be linted, then exit
      --force-default-config  Ignore all config files, use built-in defaults
  -h, --help                Print help

Local Development

cargo check          # fast compile check
cargo test           # run all tests (2,700+)
cargo run -- .       # lint current directory

# Quality checks (must pass — zero tolerance)
cargo test config_audit     # all YAML config keys implemented
cargo test prism_pitfalls   # no missing node type handling

# Benchmarks
cargo run --release --bin bench_nitrocop          # full: setup + bench + conform
cargo run --release --bin bench_nitrocop -- bench # timing only

How It Works

1. Config resolution — Walks up from target to find .rubocop.yml, resolves inherit_from/inherit_gem chains, merges layers
2. File discovery — Uses the ignore crate for .gitignore-aware traversal, applies AllCops.Exclude/Include patterns
3. Parallel linting — Each rayon worker thread parses files with Prism (ParseResult is !Send), runs all enabled cops per file
4. Cop execution — Three check phases per file: check_lines (raw text), check_source (bytes + CodeMap), check_node (AST walk via batched dispatch table)
5. Output — RuboCop-compatible text format or JSON